1. Field of the invention
The present invention concerns a method and a device for coding printed circuit boards adapted to be connected into a subrack adapted to be mounted in a cabinet or a rack. Each board is guided by slideways which are part of a plurality of parallel slideways perpendicular to a backplane of the subrack and has a front panel in the form of a wall perpendicular to its plane adapted to close part of an open front side of the subrack when the board is inserted into the latter.
The method may be used to prevent incorrect insertion of boards into a standardized subrack forming part of a telecommunication installation cabinet or rack. A particular object of the invention is to prevent insertion into the same subrack of boards from different and incompatible families although part of a common type of product and having the same functionality.
2. Description of the prior art
The cabinets or racks used in telecommunication installations each comprise a number of parallelepiped-shape subracks with standardized dimensions and having a rear wall called the "backplane" in the form of a printed circuit board carrying multipin connectors adapted to receive mating connectors each carried by one plug-in printed circuit board and connected to the circuitry on the board, the front side of the subrack opposite the backplane being open to enable the removable insertion of a plurality of parallel boards each in a vertical plane perpendicular to the backplane. Each board has to be inserted at a specific location of the subrack to assure not only correct mechanical coupling but also the electrical coupling implementing the connections needed between the backplane circuit and the board circuit. The boards are guided as they are inserted in order to align the connectors carried by the boards exactly with the respective connectors fixed to the backplane. To this end the subrack is fitted with parallel slideways perpendicular to the backplane.
Each printed circuit board carries a so-called "front panel" perpendicular to its main plane fixed to the edge of the board parallel to that carrying the connector(s) to be plugged into the respective connector(s) fixed to the backplane. The front panel faces the open side of the subrack during and after insertion of the board into the subrack and can carry information concerning the correct location of the board in the subrack. However there is still some risk of misreading or misinterpretation of this information or of handling errors subsequent to such reading, resulting in the incorrect insertion of boards into the subrack.
One known way to eliminate the risk of error in the individual insertion of each board is to adopt an individual coding method whereby for each board there is a coding member fixed to or forming an integral part of at least one connector fixed to the backplane and a respective coding member fixed to or forming an integral part of at least one connector fixed to the board, the two coding members being rendered selectively engagable plug-and-socket fashion by coding achieved (for example) by fitting removable "polarizer keys" to obstruct predetermined openings of a series of identical openings in one member and removing from the other member projecting members in corresponding relationship to the position of the openings closed off by the polarizer keys and part of a series of members adapted to enter the openings of said series.
Adapting this individual coding method to discriminating between different board families for the same type of product and having the same functionality but which are nevertheless incompatible with each other, in other words which represent different technical and/or electrical versions (for example, ECL technology boards and CMOS technology boards), would have the disadvantage of requiring modification to the coding of all connectors fixed to the backplane. One consequence of this would be to rule out the interchangeable use of different families of boards in a given subrack without modifying again the coding of all the backplane connectors, which would require much dexterity and be somewhat time-consuming, once again with a non-negligible risk of errors.
A specific object of the invention is to obviate the disadvantage just mentioned by enabling associative coding of boards according to the family of which they are part, so retaining the interchangeability of boards within a complete family, without modifying the individual coding of the connectors fixed to the backplane or the connectors fixed to the boards.